Hydraulically driven working vehicles have been known in which a hydraulic travel pump and a working machine hydraulic pump are driven by an engine, a travel motor is rotated by pressurized oil, discharged from the hydraulic travel pump to drive a drive wheel for traveling, and a working machine cylinder is extended and contracted by pressurized oil, discharged from the working machine hydraulic pump, to actuate a working machine. As a hydraulic circuit for the above-described hydraulically driven working vehicles, a hydraulic circuit disclosed in, for example, Japanese Unexamined Patent Publication No. 57-208349 has been known. In the hydraulic circuit, a hydraulic travel pump and a hydraulic travel motor are subjected to a closed-circuit connection by first and second main circuits, and the capacity of the hydraulic travel pump (hereinafter, the capacity shows a discharge amount per one rotation, cc/rev) is changed to determine a traveling speed. At this time, pressurized oil, discharged from the working machine hydraulic pump, is supplied to a working machine cylinder by a working machine valve, and when the working machine valve is placed in its neutral position, the discharged oil is supplied to one of the first and the second main circuits. In this hydraulic circuit, the pressurized oil, discharged from the working machine hydraulic pump, is supplied to the hydraulic travel motor, and the hydraulic travel motor rotates at a speed which is higher than the rotational speed corresponding to a maximum discharge amount of the hydraulic travel pump, thereby traveling the vehicle at a high speed.
However, this hydraulic circuit requires a first directional control valve, for supplying the discharged oil to one of the first and second main circuits; a manually-operated second directional control valve; and a third directional control valve, which is placed in a drain position when pressurized oil to be supplied increases to a set pressure or higher. In addition, this hydraulic circuit requires a complicated mechanism for switching the first directional control valve, by transmitting a movement of an operation member for changing the discharge direction of the hydraulic travel pump to the control valve, and a mechanism for switching the second directional control valve; whereby the structure of the circuit becomes very complicated. Incidentally, the third directional control valve prevents the pressurized oil, discharged from the working machine hydraulic pump, from being supplied to the first and second main circuits when the pressure of the first main circuit or the second main circuit is the set pressure or higher, i.e., when a traveling resistance is high and the hydraulic travel motor rotates at a low speed. In addition, in this hydraulic circuit, if the working valve is changed from its neutral position to an operating position when the traveling resistance is low and the hydraulic travel motor is rotating at a high speed, the working machine hydraulic pump actuates the working machine, so that the hydraulic travel motor is not supported by the working machine hydraulic pump, and a constant traveling speed cannot be obtained. For this reason, it is dangerous for an operator to actuate the working machine while traveling, because the speed suddenly changes.
In addition, for example, when earth and sand are loaded on a bucket by a loader, and the loader approaches a dump truck at a slow speed, a problem arises in that the rotational speed of the hydraulic travel motor increases because the motor is supported by the working machine hydraulic pump, so that it becomes difficult for the operator to control the loader so as to approach the dump truck at a low speed.
In addition, when digging resistance is high during digging with the working machine, the working machine can stop. Then, one of the following operations is required: the working machine is operated to decrease the digging resistance; the vehicle is moved forwardly by imparting a load to the hydraulic travel motor; or the vehicle is moved backwardly. This creates a problem in that the operations required by the operator increase and cause fatigue, and the amount of work performed decreases.
Further, since the closed circuit is used, it becomes necessary to supply a constant amount of oil to the first main circuit or the second main circuit in order to prevent cavitation, and a problem arises in that energy is lost.
As an another embodiment, Japanese Unexamined Patent Publication No. 5-106245 has been known. According to the Official Gazette, a self-propelled working vehicle having an HST hydraulic travel device includes a variable displacement hydraulic pump, and a variable displacement hydraulic travel motor which is subjected to a closed circuit connection to the pump by a pair of main pipes, and obtains a traveling force using the output torque of the hydraulic motor. The self-propelled working vehicle decreases the discharge capacity of the variable displacement hydraulic motor until the detected operation speed of a front working machine reaches a predetermined value at least when the detected driving pressure of the front working machine hydraulic cylinder is a predetermined value or higher. Therefore, the traveling torque is reduced when a large front driving force is required; and a lifting force, which is larger by the amount of the reduction of the traveling torque, can be obtained so that the front working machine positively starts operation in any type of earth and sand.
However, although it is described that when the digging resistance is high during digging with the working machine, the discharge capacity of the hydraulic travel motor is reduced to decrease the traveling tractive force, and the decreased engine output is used to increase the lifting force oil pressure for increasing the lifting force, the lifting force of the working machine is not increased, so that the force available for crushing a rock bed becomes weaker by the amount of the reduction of the traveling tractive force, and the digging force cannot be increased.
In addition, since the closed circuit is used, it becomes necessary to supply a constant amount of oil to the closed circuit in order to prevent cavitation, and a problem arises in that energy is lost.